Fix blog category selection and empty state rendering

src/TaxBaik.Web/Pages/Admin/Blog/Create.cshtml.cs

@@ -3,12 +3,13 @@ using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
 using TaxBaik.Application.DTOs;
 using TaxBaik.Application.Services;
+using TaxBaik.Domain.Entities;
 using TaxBaik.Web.Services;
 
 namespace TaxBaik.Web.Pages.Admin.Blog;
 
 [Authorize(AuthenticationSchemes = AdminAuthDefaults.Scheme)]
-public class CreateModel(BlogService blogService) : PageModel
+public class CreateModel(BlogService blogService, CategoryService categoryService) : PageModel
 {
     [BindProperty]
     public CreateBlogPostDto Input { get; set; } = new()
@@ -17,14 +18,47 @@ public class CreateModel(BlogService blogService) : PageModel
         Title = string.Empty
     };
 
-    public IActionResult OnGet() => Page();
+    public IReadOnlyList<Category> Categories { get; private set; } = [];
+
+    public async Task<IActionResult> OnGetAsync(CancellationToken ct)
+    {
+        Categories = (await EnsureCategoriesAsync(ct)).ToList();
+        if (Categories.Count > 0)
+            Input.CategoryId = Categories[0].Id;
+
+        return Page();
+    }
 
     public async Task<IActionResult> OnPostAsync(CancellationToken ct)
     {
+        Categories = (await EnsureCategoriesAsync(ct)).ToList();
         if (!ModelState.IsValid)
             return Page();
 
         await blogService.CreateAsync(Input, ct);
         return RedirectToPage("/Admin/Blog/Index");
     }
+
+    private async Task<IEnumerable<Category>> EnsureCategoriesAsync(CancellationToken ct)
+    {
+        var categories = (await categoryService.GetAllAsync(ct)).ToList();
+        if (categories.Count > 0)
+            return categories;
+
+        var defaults = new[]
+        {
+            ("사업자 세무", "business-tax", 1),
+            ("부동산 세금", "real-estate-tax", 2),
+            ("종합소득세", "income-tax", 3),
+            ("부가가치세", "vat", 4),
+            ("가족자산·증여", "family-asset", 5)
+        };
+
+        foreach (var (name, slug, sortOrder) in defaults)
+        {
+            await categoryService.CreateAsync(name, null, ct);
+        }
+
+        return await categoryService.GetAllAsync(ct);
+    }
 }